The present invention relates to a method for measuring a diameter of a single crystal ingot which is an ingot grown and pulled up from a molten liquid of a polycrystal substance in a single crystal pulling apparatus in accordance with the Czochralski (CZ) method.
The single crystal pulling apparatus is an apparatus for growing and pulling up a single crystal ingot from a molten polycrystal substance. This apparatus comprises a main chamber wherein the single crystal is grown, and in this main chamber are housed a crucible for containing the polycrystal substance, a heater arranged to surround the crucible, and a heat insulator arranged to surround the heater. Beneath the main chamber are provided mechanisms for rotating the crucible about the center line thereof to control the uniformity of the heat flux in the molten polycrystal substance during the crystal growing process. Rising above the main chamber is a towering structure of a crystal pull chamber which is an upright, substantially cylindrical enclosure terminating in an upper structure of a crystal pulling mechanism. The single crystal ingot which is grown and raised high in the main chamber is brought up into the pull chamber.
In the single crystal pulling apparatus, the single crystal is caused to grow from the lower tip of a seed crystal which is attached to the lower end of a pull device such as a wire and which is dipped in a polycrystal substance such as silicon that is melted down by the heater. As the seed crystal is rotated and brought up higher slowly, the desired single crystal is grown longer from the seed tip, and the diameter of the single crystal gets larger as the length of the the single crystal increases until the diameter becomes a predetermined size. Thus, the head portion of the single crystal ingot becomes conical in shape and the main body portion becomes cylindrical. The portion of the ingot where the conical head portion turns cylindrical is called shoulder portion. The main purpose of measuring the diameter of the growing ingot is to maintain the diameters of the cylindrical portion as close to the predetermined size as possible to thereby obtain an ingot of uniform diameter.
The diameter of the growing single crystal ingot at the solid-liquid interface is measured in a real time manner by means of an optical sensor such as an image sensor, ITV, and the like. In order to obtain accurate values of the diameter of the single crystal ingot, it is necessary that the distance between the head (or the eye) of the optical sensor and the surface level of the polycrystal melt is maintained constant, or that the values measured by the optical sensor are corrected in response to the increase of the distance therebetween.
However, in the single crystal pulling apparatus, the surface level of the polycrystal melt in the crucible shifts downward relative to the wall of the crucible, as the single crystal ingot is pulled up. Therefore, in order to maintain constant the distance between the optical sensor and the surface level of the polycrystal melt, it is necessary that the crucible is raised to compensate for the fall of the melt surface level. For this reason in the conventional single crystal pulling apparatuses the crucible is supported in a manner such that it can be shifted vertically at arbitrary velocities, and thus during the single crystal pulling operation, the crucible is raised by means of a drive means in a manner such that the melt surface is always kept stationary relative to the optical sensor.